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162                                ALASKAN GLACIER STUDIES
thus visibly demonstrable that the glacier had overridden the gravels, eroding them, as shown by the broadly-truncated stratification of the gravel, and with erosion most pronounced in the centre of the valley because the eroded surface sloped that way. Gilbert had detected the overriding of these gravels in 1899,1 before recession had revealed the site where the glacier itself rested on the gravels in 1905 (PI. LXXVIII, A), observing the smooth curves eroded by flowing ice, truncating stratified gravel deposits. He also noted the fact, which we likewise observed, that there was a veneer of till and scattered erratic blocks resting on the surface of the truncated gravels. The overridden gravels extend outside of Hidden Glacier valley far down into Russell Fiord, proving a former great extension of the glacier, during whose advance erosion had sculptured but not removed gravels of earlier deposit.
In 1905 a stagnant, outlying mass of moraine-protected ice lay upon the gravels over an eighth of a mile from the glacier, showing gradations of kame topography, with various stages of melting according to the distance from the main glacier. It was a lateral moraine mass whose melting would later produce a superposition of hummocky topography upon earlier, ice-eroded gravels.
Thus in 1905 the terminus of Hidden Glacier showed clearly the two interesting phenomena of a glacier margin resting upon old gravels which were sculptured but not wholly removed, and the middle of a glacier with its terminus buried beneath modern gravels which were still being deposited upon it.
Condition in 1906. The recession of Hidden Glacier from its former advanced position, concerning which we have specific evidence between 1899 and 1905, continued at least until June, 1906. At that time the Geological Survey party visited Seal Bay and took a photograph of the Hidden Glacier from near its mouth, but seeing that the glacier was in essentially the same position and condition as in 1905, and having other objects in view for the season's work, no further study was given Hidden Glacier at that time.
Condition in 1909. On July 10, 1909, when the National Geographic Society party rounded the point which forms the north side of Seal Bay, the formerly Hidden Glacier burst upon our view with an ice front so high and so near that at first we thought it had become tidal (PI. LXXVHI, B). It no longer deserved the name Hidden, for it boldly projected out in its valley so far that it formed a prominent feature in the landscape from all points of view where one could look into Seal Bay. Hidden Glacier, stagnant in 1899, 1905, and as late as July 6, 1906, and showing striking evidence of notable recession between 1899 and 1905, had suddenly changed its condition, and between July 6,1906, and July 10,1909, had pushed its front forward fully two miles, and to within a little over a quarter of a mile of the sea. Where the ice front stood in 1905 and 1906 there was a thickness of about eleven hundred feet of ice in 1909 (Fig. 14), as shown on our detailed topographic map (Map 5), Our most important photographic sites, selected in 1905 for the purpose of showing changes in the glacier front as it continued to recede, were deeply buried beneath the glacier. The formerly-overridden gravel terraces were covered by ice and the glacier had completely overridden the pitted outwash gravel plain (PL LXXXV). Its front lay nearly a mile beyond the outermost limit of kettle development in 1905.
Two of our photographic sites of previous years could be reoccupied because tliey lay far enough down the valley to be beyond the reach of the advancing ice front. One of
i Gilbert, G. K, Glaciers and Glaciation, Hairiman Alaska Expedition, Vol. 3, 1004, p. 58.